1. Field of the Invention
The present invention relates to an image area extracting method and is suitably applied, for example, to the case of forming a key signal (a mask image) which is necessary at the time of an image synthesizing process among the works related to video production.
2. Description of the Related Art
Techniques to extract an object from an image are conventionally well-known as methods for special effects. The techniques to extract an object are the mask image hand painting (video mat) method, the method using chroma key processing and luminance key processing, and the rough contour assigning and thinning (Object Extracting Method for Image Mixing, by Seiki Inoue, The transactions of the Institute of Electronics, Information, and Communication Engineers D-II vol. j74-D-II pages 1411-1418, 1991).
The mask image hand painting method, in which an operator assigns an object area by hand painting in utilizing a painting tool such as a tablet, is a method capable of inputting the boundary of an object precisely.
On the other hand, the chroma key processing or the luminance key processing is the method to divide the area with a specified color or specified luminance by performing threshold processing on color information or luminance information of the input image. In this method, only assigning of the specified color or the specified luminance by the operator makes it possible to execute a uniform processing automatically to all picture elements.
Moreover, in the method of rough contour assigning and thinning, the rough area including the contour of the object is assigned by an operator to obtain the shape of object by thinning the rough area based on the intensity of edge so as to match this rough area to the contour of object automatically.
However, in the chroma key processing and the luminance key processing, an input image of which its characteristics greatly differ according to the position in one screen is processed in accordance with single threshold value to the pixel value of the entire image, the chroma key processing and the luminance key processing cannot be applied to the image with the arbitrary background. Therefore, they can be applied only to the specified input image taken with the uniform background for the purpose of synthesizing.
The rough contour assigning and thinning method is the method to assign the rough area and which is capable of extracting the object area from the uneven background. However, since this method is the processing only dependent upon the edge intensity, the correct result cannot be obtained in the area with an intense edge which is not the contour of the object, and there are cases where the accuracy is worse as compared with the video mat method. Also, in the case where the characteristics of background differs according to the position on the image, since single area extracting method is used, it is impossible to cope with the arbitrary background thoroughly.
Accordingly, in these methods, it was only video mat method which was capable of extracting the object area accurately from the optional background. However, because the video mat method requires the process to assign the object area per one pixel, it requires much time and skill and there arises a problem that its operational efficiency is not favorable.